(1) Field of the Invention
The present invention relates to a solid-state laser oscillator and a solid-state laser material for use therein, and a method for manufacturing such material, and more particularly to an Er:YVO.sub.4 (erbium: yttrium orthovanadate) laser oscillator, a solid-state laser material used therein, and a method for manufacturing the same.
(2) Description of the Related Art
In recent years, there is a high demand for a shortwave laser in the field of applications such as writing/reading on optical disks and laser-machining or processing. Today, a gas laser is in use as a shortwave laser which has a problem associated with its maintenance and increased device scale or size, which leads to a demand for the development of a solid state type device. With this as background, research is in progress for the development of a solid state-shortwave laser device, and there are reports on such laser devices. In one of such devices, with the solid-state laser including an Nd.sup.3+ activator, a laser of 1064 nm by energy transitions .sup.4 F.sub.3/2 .fwdarw..sup.4 I.sub.11/2 of Nd.sup.3+ ions (neodymium ions) and a non-linear optical crystal are combined to provide a second harmonic generation of 532 nm, and in another of such devices, the same shortwaves are provided by a laser crystal which itself has non-linear optical effects. Also, in an up-conversion laser device which is being recently researched, such ions as Ho.sup.3+ ions and Pr.sup.3+ ions are mixed in glass or fluoride crystal whereby laser oscillation of a 500 nm band is obtained ("Applied Physics" of 1992, Vol. 61, 43 p). Other reports include a report in which, with Nd.sup.3+ ions as activators, the oscillation takes place at a 950 nm band by utilizing energy transitions .sup.4 F.sub.3/2 .fwdarw..sup.4 I.sub.9/2 of Nd.sup.3+ ions (for example, "IEEE Journal of Quantum Electronics" of 1987, Vol. QE-23, 605 p), and a report on a recently developed variable wavelength laser device representative of Ti:Al.sub.2 O.sub.3 laser ("O plus E" of 1988, No. 102, 119 p).
As a laser in which a host material is YVO.sub.4, the most common is a 1.06 .mu.m laser device with an Nd activator. Other reports include one on a Tm activated laser ("Optics Letters" of 1992, Vol. 17, 189 p).
As a host material in a laser device with an Er activator, the reported examples include glass (Japanese Patent Application Kokai No. S63-81994), YLiF.sub.4 ("Electronics Letters" of 1989, Vol. 25, 1389 p), Y.sub.3 Al.sub.5 O.sub.12 and Y.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12 ("Journal of Applied Physics" of 1991, Vol. 70, 7227 p).
However, the reported shortwave solid-state laser devices as above have encountered the following problems. That is, since the harmonic generation is produced by combining the laser oscillation and the non-linear optical effects with the wavelength conversion step being processed through, there have occurred substantial conversion losses. Also, in the up-conversion laser, the oscillation spectral line width is large when the glass is a host material, and the growth of fluoride crystal, especially for the growth environment, requires more care than that for the growth of the oxide crystal. Further, in the laser which utilizes the energy transitions .sup.4 F.sub.3/2 .fwdarw..sup.4 I.sub.9/2 of Nd.sup.3+ ions, since this is a three-level laser, there are problems such as high oscillation threshold, low oscillation efficiency, and low oscillation strength. In the variable wavelength laser, often a gas laser which is large and has maintenance problem is used as an excitation source and this inevitably makes the overall laser system large and complex. With respect to the laser which utilizes .sup.4 F.sub.3/2 .fwdarw..sup.4 I.sub.9/2 of Nd.sup.3+ ions, there has been found no report wherein YVO.sub. 4 crystal capable of performing highly efficient oscillation is used as a laser host material. This may be because, with the YVO.sub.4, the central wavelength of the luminescent characteristics of 950 nm band of Nd in the crystal deviates and overlaps the wavelength band in which absorption coefficient is large, resulting in excited state absorption.
An Er glass laser or an Er:YLiF.sub.4 laser making use of an Er activator is a shortwave laser but, where the host material is glass, there are such problems that an oscillation wavelength profile is broad and that, since glass containing high concentration Er is difficult to produce, a large glass laser is required resulting in an increase in the size of the device. On the other hand, where the host material is YLiF.sub.4, a problem is accompanied in the crystal growth as it requires extra care in its growth environment.
The second aspect of the invention relates to a solid-state laser device operable in an eye-safe region.
In recent years, there is a desire for the realization of a laser used in an atmosphere to be operable in an eye-safe region, and development of such a laser device is in progress. As the lasers that operate in eye-safe regions, the reports known to the inventors include a Tm laser operated in 1.87-2.14 .mu.m ("Optics Letters" of 1990, Vol. 15, 486 p); an Ho laser operated at 2.1 .mu.m ("Optics Letters" of 1990, Vol. 15, 320 p); an optic parametric oscillation of 700-2200 nm at 1.6 .mu.m by a second harmonic generation (532 nm) of Nd laser and a nonlinear crystal LiNbO.sub.3 (Extended Abstracts--The 52nd Autumn Meeting, 1992--The Japan Society of Applied Physics, 11 p-M-4).
However, the prior art eye-safe lasers have suffered from the following problems. That is, while the most appropriate wavelength for the eye-safe region is 1.5 .mu.m or its vicinity, the Tm laser or Ho laser used in the prior art deviates from such wavelength band. Also, since the optic parametric oscillation undergoes nonlinear optical effects twice, the conversion losses are inevitable and the oscillation intensity is very weak.
The oscillation wavelength of the Er glass laser is 1.5 .mu.m which is within the eye-safe region but, since the laser host is glass, there are such problems that an oscillation wavelength profile is broad and that, since glass containing high concentration Er is difficult to produce, a large glass laser is required resulting in an increase in the size of the device and also in a reduced heat resistance. On the other hand, the oscillation wavelength of other Er activating lasers fall outside the eye-safe region.
The third aspect of the invention relates to a solid-state laser material and a method for manufacturing such material.
Conventionally, a YAG laser has been typically used as a solid-state laser material. The laser which has improved the YAG laser in the aspect of laser oscillation efficiency is a YVO.sub.4 laser. The reason that the YVO.sub.4 laser is highly efficient is attributable to the facts that, as compared with the YAG crystal, the YVO.sub.4 crystal has a broader light absorption spectrum with high absorption rate of excitation, and that the intensity of luminescent characteristics is great.
However, in the conventional YVO.sub.4 laser described above, the activator has employed only Nd and this has presented the problem that the oscillation cannot be obtained other than at the oscillation wavelength of 1.06 .mu.m. However, in recent years, there are demands, on one hand, for shortwave lasing for applications such as for meeting changes from a gas laser, and enhancing higher density of optical disks and higher precision in laser machining and processing and, on the other hand, for long-wave lasing for eye-safe purposes.
Conventionally, it has not been easy to grow YVO.sub.4 crystals and there have been desires for the availability of an improved crystal growth method capable of growing a single crystal of good quality.